Historically a large number of ROVs (remotely operated vehicles) have been built for subsea service and have a characteristically low ability to pick up useful payloads.
Tasks usually done by the existing remote vehicles are to carry TV cameras for viewing, using complex manipulators to operate small T-Handles on valves, or to attempt simple unplanned repairs. A characteristic inability has existed to pick up useful loads. In some cases, loads are carried by neutrally buoyant ROVs by thrusting up. This has produced the capacity to lift 20-200 lbs. under full upward thrust. This does not provide adequate stability or control to install or remove subsea components. Subsea components can be typically expected to have alignment requirements which imply the need for stability.
One concept has provided a clump weight on a single line to be landed on the ocean floor providing vertical stability and net buoyancy to the ROV itself. No trim stability is provided for that system. The trim is the front to rear elevation of the vehicle. Picking up any load in a manipulator causes a tilting of the trim. System of this type can move the attachment point of the single line and adjust the trim. On a system of this type, when a package is latched onto, the trip adjustment can be made only after the package is separated from the base structure to which it was attached.